1. Field of the Invention
The present invention relates generally to rotating magnetic disk drives and, more particularly, to a disk drive having an improved servo burst pattern that better accommodates a wide reading transducer and a wider range of transducer widths.
2. Description of the Related Art
In a conventional disk drive, each surface of each disk contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information is provided on this disk or another disk to determine the position of the head. The most popular form of servo is called "embedded servo" wherein the servo information is written on this disk in a plurality of servo sectors that are angularly spaced from one another and interspersed between data sectors around the track. Each servo sector generally comprises a track identification (ID) field and a group of servo bursts which the servo control system samples to align the transducer head with or relative to a particular data track. Each servo burst is conventionally formed from a series of magnetic transitions defined by an alternating pattern of magnetic domains.
The servo control system moves the transducer toward a desired track during a coarse "seek" mode using the track ID field as a control input. Once the transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the transducer head over that track in a fine "track follow" mode. The transducer generally reads the servo bursts to produce a position error signal (PES) that is 0 when the transducer is at a particular radial position. The position where the PES=0 may or may not be at the data track center, however, depending on the magnetic characteristics of the transducer, the arrangement of the servo bursts, and the formula used to calculate the PES.
In all drives, the write transducer's width defines the very minimum data track pitch. The servo information, therefore, must define a data track pitch that is slightly wider than the write transducer. The data track pitch is typically defined to be about 125% wider than the nominal width of the write transducer and conversely, therefore, the write transducer is about 80% of the track pitch. The exact percentage will vary somewhat from 80%, from drive to drive, since the width of the write transducer will vary from nominal due to normal manufacturing distributions.
FIG. 1 schematically represents an older drive having only two angularly successive servo burst regions 250, 350 per servo sector, only two servo burst pairs (e.g. 212, 312) per data track (e.g. 412), and an inductive head in which the same transducer 200 (inductive gap) both reads and writes data. In this particular drive, the R/W transducer 200 records "100%" servo bursts 212, 312 that stretch from data track center to data track center. Each burst, therefore, is 100% as wide as the data track pitch. The servo bursts associated with each data track are typically designated as an A burst and a B burst with reference to their corresponding angular servo burst region 250, 350. As shown in FIG. 6, the A and B bursts 212, 312 define a burst pair centerline 412 that coincides with the data track centerline (not separately numbered). When the R/W transducer 200 is on track center in such arrangement, it detects equal signal amplitudes from the two circumferentially successive, radially offset A and B bursts such that A=B. If the R/W transducer 200 shifts one way or the other away from the burst pair centerline 412, an inequality exists between the signal amplitudes, such that A.noteq.B. Assuming everything is linear, the inequality is proportional to the mechanical offset of the R/W transducer 200 relative to the burst pair centerline 412.
The servo pattern of FIG. 1 is subject to nonlinear regions because the R/W transducer "sees" too little. With only two bursts and one burst pair centerline per data track, the R/W transducer may be completely over one of the bursts and no longer pass over any part of the other burst if the transducer is displaced too far from the burst pair centerline. The 80% R/W head 200 of FIG. 1, for example, can only be displaced by a maximum of 40% of a data track pitch from the burst pair centerline 412 and still pass over at least a portion of both bursts 212, 312. This results in "blind spots" or "gaps" where the head position is ambiguous.
The industry subsequently added more 100% C and D bursts in order to fill the gaps between the A and B bursts. The C and D bursts are placed in "quadrature" with the A and B bursts in that the edges of the C and D bursts are aligned with the centers of the A and B bursts. With four 100% bursts A, B, C, D positioned in quadrature, there are two burst pair centerlines per data track pitch, i.e. one burst pair centerline every 50% of a data track pitch. The 80% R/W transducer 200, therefore, will always pass over an A/B pair or a C/D pair because it is always within 25% of a data track pitch from an A/B or C/D burst pair centerline.
The industry recently began using magnetoresistive heads (MR heads) which contain two separate transducers--an old-style inductive transducer for writing and a magnetoresistive transducer for reading. FIG. 2 is a schematic plan view of a typical MR head 120 having an inductive write transducer 90 and a separate, magnetoresitive read transducer 100.
An MR head 120 can advantageously recover data in disk drives of higher areal density than is possible with an inductive head. However an MR head also presents a number of disadvantages. In particular, the separate read and write transducers 90, 100 are necessarily spaced apart from one another along the length of the supporting structure known as a "slider." As a result, their radial separation varies from ID to OD as the MR head 120 is moved in an arc by a swing-type actuator.
The drive industry presently compensates for the variable radial separation between the transducers 90, 100 by "microjogging" the read head 100 relative to a given burst pair centerline by an amount corresponding to the radial displacement at that cylinder. This jogging solution generally requires separate and distinct track following procedures for reading and writing. The drive can microjog when writing data or when reading data, but it is preferable to track follow on the burst pair centerline while writing and only jog when reading so that the data is consistently written to the same location. In a typical MR drive, therefore, the read transducer 100 track follows a burst pair centerline at the "null" position where the PES=0 and the write transducer 90 records the data track offset toward the ID or the OD by the amount of radial separation between the read and write transducers 90, 100 at this cylinder. For reading, the read transducer 100 is "micro-jogged" away from the null position of the burst pair centerline where the PES.noteq.0, in order to align the read transducer 100 with the recorded data.
An MR head is sometimes called a "Write Wide/Read Narrow" head because the inductive write transducer 90 is usually wider than the magnetoresistive read transducer 100, as shown in FIG. 2. When the servo information is recorded so that the write transducer 90 is about 80% of a data track pitch for the reasons described above, the read transducer 90 is only about 66% of a data track pitch. The relatively narrow read transducer 100 physically limits the microjogging operation to one half of the read transducer's width, i.e. to about .+-.30%. As described in more detail below, however, the actual linear microjogging maximum is even less than 30%, because the magnetoresistive read transducer 100 has an uneven "microtrack profile" (i.e. is not uniformly sensitive across its width) and is subject to "side reading" (i.e. is sensitive to nearby transitions not actually under the transducer). Because of these factors, the typical 66% magnetoresistive read transducer 100 can only be microjogged by about .+-.20% of a data track pitch and still provide a servo signal that varies in adequate linear proportion to displacement from a burst pair centerline.
The drive industry increased the number of burst pair centerlines per data track pitch in order to reduce the linearity problem caused by the narrow linear width of the magnetoresistive read transducer 100. The additional burst pair centerlines are added by packing more servo bursts into the circumferential or radial dimensions of the disk. Adding more servo bursts in the radial dimension is generally preferred because it does not increase the angular width of the servo wedges and thereby reduce the area available for storing data. Adding more servo bursts in the radial dimension does, however, require bursts that are narrower than 100% of a data track pitch. For example, using four 2/3 track pitch bursts A, B, C, D on 1/3 track pitch offsets to create a 1/3, 1/3, 1/3 pattern of three burst pair centerlines per data track pitch ensures that the read head is always within 1/6th of a data track pitch (16.67%) from a burst pair centerline, i.e. well within the linear range of about .+-.20% for a typical magnetoresistive read transducer. This solution is not without cost.
The narrow bursts and additional burst pair centerlines reduces one linearity problem, but creates a new linearity problem by reducing the width of the servo bursts relative to the width of the read transducer while putting neighboring, radially adjacent servo bursts closer together. In particular, when reading a pair of narrow bursts with a wide transducer, a neighboring servo burst may undesirably affect the signal amplitude detected by the read transducer 100 in a given angular servo burst region. This is particularly true as the read transducer 100 is microjogged away from a burst pair centerline and moved closer to the neighboring, radially adjacent servo burst. Stated another way, as the read head is microjogged away from the B burst of an A/B burst pair centerline, it may begin to "see" the radially adjacent B burst. Given the 1/3, 1/3, 1/3 pattern described above, for example, the read head may be microjogged 15% of a data track pitch from the A/B burst pair centerline, involving a first B burst, and begin to see more and more of a second, radially adjacent B burst, thereby distorting the B portion amplitude signal used to provide the PES that is typically defined as (A-B)/(A+B).
Accordingly, there is a need for a disk drive that reduces or eliminates the detrimental effects of side reading. The present invention is applicable to a read transducer of any type, but is especially helpful in the context of MR heads which are necessarily jogged during operation, are often manufactured and used in a wide range of physical width distributions to keep costs down, are subject to increased side reading due to shielding, and are more likely to be used with narrow servo bursts needed to define additional burst pair centerlines.